GPUModel setup of Mersenne Twister only works in some cases

[NicolasJPosey]

In GPUModel::setupSim, we initialize the Mersenne Twister for generating noise per vertex. However, it explicitly assumes that numVertices is >= 100 and a multiple of 100. If a simulation is run that doesn't met this requirement, an invalid kernel configuration error will be thrown. It doesn't look like it impacts running a kernel but could mask other errors.

We should look into trying to generalize this to work for all configurations (i.e. any number of vertices).